The xylene isomers are important intermediates which find wide and varied application in chemical syntheses. Para-xylene is a feedstock for terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
The proportions of xylene isomers obtained from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20-25% of a typical C8-aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization, and these usually are differentiated by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function.
Catalysts containing molecular sieves have become prominent in these approaches to xylene isomerization in the past few decades. U.S. Pat. No. 3,377,400 teaches liquid-phase isomerization and disproportionation of alkylaromatic hydrocarbons using a crystalline aluminosilicate catalyst. U.S. Pat. No. 3,856,872 teaches xylene isomerization and ethylbenzene conversion with a catalyst containing ZSM-5, -12, or -21 zeolite. U.S. Pat. No. 4,957,891 discloses a catalyst for the isomerization of a mixture of xylenes and ethylbenzene comprising a platinum-group metal, gallium-substituted pentasil zeolite and zirconia-alumina matrix. U.S. Pat. No. 4,962,258 discloses a process for isomerization of a major amount of xylenes and minor amount of ethylbenzene over gallium-containing, crystalline silicate molecular sieves as an improvement over aluminosilicate zeolites ZSM-5, ZSM-12, and ZSM-21. U.S. Pat. No. 6,872,866 discloses a liquid-phase process using two catalysts comprising beta zeolite and low Si/Al2 MTW for the isomerization of xylenes and ethylbenzene.
In contrast to the known art for isomerizing xylenes with effective conversion of ethylbenzene, the present invention features high retention of ethylbenzene while isomerizing xylenes. This approach enables effective further processing of the isomerized product with selective conversion of ethylbenzene to paraxylene. Alternatively, the product from the present process could be separated to recover ethylbenzene as well as para-xylene and/or other xylene isomers. In yet other embodiments, ethylaromatics can be preserved when isomerizing alkylaromatic feed mixtures containing higher alkylaromatics such as ethyltoluene.